This invention relates to a method of manufacturing a compound semiconductor thin film of a metal sulfide for photoelectric devices or solar cells, and solar cells using the compound semiconductor thin film as an n-type semiconductor layer.
Compound semiconductor thin films consisting of metal sulfides, such as cadmium sulfide (CdS), zinc sulfide (ZnS), lead sulfide (PbS), copper sulfide (CuS), mercury sulfide (HgS) and so on, are extensively used for photoelectric device materials in the photoelectric device industry. For example, a CdS thin film has been used to construct a photo-conductive sensor and a photo-filter by utilizing the change of its electric resistance caused by the irradiation of light on an optical filter. Also, CdS/CdTe system compound semiconductor solar cells comprised of the thin films of CdS and CdTe have been commercialized.
Many of these compound semiconductor thin films are manufactured by a sputtering method or a vacuum evaporation method, in which source materials are evaporated under reduced pressure and thin films are formed on a substrate. By using any of these methods, the desired thin film properties for photoelectric device materials can be obtained. However, since these methods have to be conducted in a vacuum chamber, they are not suitable for a high-speed continuous deposition of thin films with large surface area and with uniform thickness. If the investment for enabling the large scale deposition of thin films by any these method were made, the amount for this large and complex equipment would be prohibitively high.
The chemical bath deposition method is a method with which compound semiconductor thin films of large surface area can be more inexpensively manufactured than by the above methods. For example, for manufacturing CdS thin film, a substrate is immersed in an aqueous solution containing both cadmium-containing compound such as cadmium acetate and sulfur-containing compound such as thiourea, and a CdS layer is deposited on the substrate. When the CdS layer is heat treated, it becomes dense and thus a CdS thin film is formed.
With this method, a CdS thin film of large surface area can be manufactured, but the obtained CdS thin film is not uniform and the process lacks in reproducibility since the CdS layer cannot be uniformly deposited.
As another method of depositing a thin film, Pyrosol process, based on the pyrolysis of an aerosol produced by ultrasonic spraying method, has been reported (Thin Solid Films. vol. 77. pp. 81-90(1981) ). This research is mainly directed to the deposition of metal oxides (In.sub.2 O.sub.3, Fe.sub.2 O.sub.3, SnO.sub.2 etc.) on a glass substrate by the pyrolysis using a metal organic compound as a source material.
In this report, it is described that the deposition of other materials such as metal sulfides is also possible by the above-mentioned method. But, concerning the deposition of the metal sulfides such as CdS, it is only abstractly described that two (2) compounds, providing the metal and the sulfur respectively, are needed as source materials for such coating.
Furthermore, this method of the deposition of the metal sulfides is considered to have various disadvantages, such as that it is very difficult to select these two source materials having the same thermal decomposition temperature and that it is very difficult to control the concentration of the provided metal and sulfur at the constant ratio.
The printing and sintering method has been proposed with which compound semiconductor thin films of large surface area and with uniform thickness can be continuously manufactured by using inexpensive equipment with good reproducibility. In this method, a paste is prepared by dispersing fine particles of a compound semiconductor and then the paste is coated on a substrate in the form of a film by a screen printing method, dried and sintered in the furnace on a continuous moving belt.
With this method, it is also possible to perform patterning during thin film formation. Consequently, this method is now practically used for the manufacture of CdS/CdTe system compound semiconductor solar cells which are comprised of the sintered thin films of CdS and CdTe (Japanese Patent Publication No. Sho 56-28386).
In this case, a CdS thin film is manufactured by the following process. First, a paste is prepared by dispersing fine particles of CdS, added with cadmium chloride (CdCl.sub.2) as a reducing agent of the melting point, in a dispersing solvent such as propylene glycol and then the paste is coated on a substrate, dried and sintered and thus the Cds thin film is manufactured. However, this method has the following disadvantages.
Since the required sintering temperature is as high as about 700.degree. C., a conventional inexpensive soda lime glass plate cannot be used as a substrate, it is necessary to use a substrate having both a high heat resistance and chemical stability, such as a ceramic sheet of alumina of high purity or a barium borosilicate glass, both of which are known as very expensive materials. It is not suitable for high speed mass production, since the sintering period required is often more than two (2) hours.
Moreover, the sintering has to be performed in an inert gas atmosphere, such as in nitrogen gas, and by accommodating a dried substrate in an expensive ceramic case in order to suppress the quick evaporation of CdCl.sub.2. Additionally, since a source material of CdS powder can only be pulverized into fine particles of 2 to 4 .mu.m, it is not possible to manufacture a film of thinner than 2 to 4 .mu.m, which is a diameter of pulverized CdS fine particles. Normally, CdS film as per manufactured by this method has a thickness of 20 to 60 .mu.m. Its surface is often irregular and has a number of void spaces inside of the film. Consequently, it is not possible to obtain a thin film of uniform quality.
Since a compound semiconductor thin film which is manufactured by the printing and sintering method is relatively thick with a number of void spaces insides, it has a low light transmittance. Consequently, when this thin film is used as an n-type semiconductor layer of a solar cell, the solar cell cannot have an acceptable photoelectric conversion efficiency.
Recently, another method of manufacturing a compound semiconductor thin film of a metal sulfide is proposed in which a metal organic compound containing at least one metal-sulfur bond is thermally decomposed. This method is advantageous in that an inexpensive soda lime glass plate can be used as a thin film forming substrate since film forming temperature is lower than that required for the sintering in the printing and sintering method.
Example of the concrete method of manufacturing the thin film by using this method is as follows. First, the metal organic compound layer is formed on a substrate and then a thin film of a metal sulfide is formed on the substrate by thermally decomposing the metal organic compound in an inert gas atmosphere or in a mixed gas atmosphere of an inert gas and hydrogen sulfide(H.sub.2 S) (Japanese Laid-Open Patent Publication Nos. Sho 61-166979 and Sho 61-166978).
However, this method has various disadvantages such as the metal organic compound is not completely decomposed in the inert gas atmosphere and as a result, black organic ingredients containing carbon or carbon compound remain in the thin film as performed.
In order to solve the above problem, another concrete method is proposed with which a solution of the metal organic compound is coated on the substrate by, for example, spin coating method and a metal sulfide thin film is formed on the substrate by thermally decomposing the metal organic compound in oxygen containing atmosphere. (Japanese Laid-Open Patent Publication No. Sho 62-146276).
Another concrete method is proposed with which an organic solvent solution of the metal organic compound, the solution having a viscosity of a specified range, is coated on the substrate by a printing method, dried and a metal sulfide thin film is formed on the substrate by thermally decomposing the metal organic compound. (Japanese Laid-Open Patent Publication No. Hei 8-316247).
However, any of these two (2) concrete methods have various disadvantages, such as it is very difficult to uniformly coat a solution of the metal organic compound on the substrate of large surface area and thus it was not possible for a metal sulfide thin film with large surface area and with uniform thickness to be manufactured in an industrial scale.
In these two methods, it is aimed to manufacture a thin film of high purity with few residual ingredients by sufficiently oxidizing the metal organic compound during the thermal decomposition in oxygen containing atmosphere and thus removing all ingredients other than metal and sulfur by vaporization of the ingredients.
However, it was very difficult to manufacture a thin film of acceptable high purity since it is difficult to completely oxidize and thermally decompose the whole metal organic compound especially which is present in the proximity of the surface of the substrate among the coated layer of the organic solvent solution. It was often the case that some amount of impurities remained.
In addition to these methods, another concrete method of manufacturing a compound semiconductor thin film by thermal decomposition is proposed with which a paste containing the metal organic compound containing at least one metal-sulfur bond is coated on a source substrate and the metal organic compound is vaporized by heating the source substrate. Then, the gaseous metal organic compound is thermally decomposed by contacting the gaseous metal organic compound at the heated surface of a thin film forming substrate which is placed opposite to the proximity of the source substrate, and a metal sulfide thin film is formed on the thin film forming substrate. (Japanese Laid-Open Patent Publication Nos. Hei 9-74065 and Hei 10-4206 and U.S. Pat. No. 5,714,391).
However, this method has various disadvantages, such as manufacturing cost of the thin film is very expensive since the source substrate is necessary in addition to the thin film forming substrate and also a complicated coating process of the paste on the source substrate is necessary.
Moreover, it is difficult to uniformly contact the gaseous metal organic compound at the surface of the thin film forming substrate and to supply the center part of the surface of the thin film forming substrate with sufficient amount of oxygen. Consequently, it was difficult to manufacture the thin film with uniform thickness, high purity and large surface area by this method.